This invention relates to an electrical pittingproof rolling bearing having an insulating film on its bearing ring surface.
Heretofore, for rolling bearings such as ball bearings, in order to prevent electrolytic corrosion phenomena produced between the bearing rings and the rolling elements, an insulating film has been formed on a bearing ring surface to shut off current flowing into the bearing rings from outside (Japanese utility model publication 2-85016).
As electrical pittingproof rolling bearings having such an insulating film, one having a film of polyphenylene sulfide resin (hereinafter abbreviated as PPS) containing glass fiber is disclosed in Japanese patent publication 3-277818 and one using a thermoplastic resin such as polyamide resin or PPS as a covering material is disclosed in Japanese utility model publication 5-89953.
Since they have a film made of a resin having high insulating properties formed on the surfaces of their bearing rings, they have a stable electrical pittingproof function. Also, since the film is integrally formed on the bearing rings by injection molding, high moldability is achieved and the film can be formed on the bearing rings at a low cost.
But if such conventional electrical pittingproof rolling bearings are used at such a high temperature as over 100xc2x0 C. for a long time, the insulating film might be creep-deformed under bearing loads. In other words, there is a problem that if they are used for a long time at high temperature, interference of the bearing decreases with time.
In mounting a bearing on a shaft or in a housing, the shaft on which rotary load acts is normally fixed by xe2x80x9ctight fitxe2x80x9d. For such a xe2x80x9ctight fitxe2x80x9d, it is necessary to give an interference to the fitting surface between the bearing ring and the shaft or housing.
Generally, a maximum interference is 1/1000 or under of the shaft diameter or the bearing outer diameter. If the interference decreases with time, relative movement occurs at the bearing fitting surface in radial, axial and rotational directions, so that undesirable problems such as wear of the fitting surfaces may arise.
Reducing the change of interference with time by forming the insulating film of a small thickness is theoretically possible. But it is not easy to form an insulating film of a resin having a thickness of e.g. 0.2 mm with a uniform thickness. This is because high melt-viscosity of the resin can cause insufficient filling of resin in a mold. In fact, if the thickness of the insulating film is 0.3 mm or less, it tends to crack, and forming it to a uniform thickness is difficult. Thus it was impossible to practically suppress change in the interference with time by thinning the insulating film.
As an example of electrical pittingproof rolling bearings used for a long time under large loads, as shown in FIG. 4, there are electrical pittingproof rolling bearings 12, 13 used with a motor 11 of an electric railroad car and its rotating force transmitting device. Rolling bearings applicable to such use are required.
Also, by adding glass fiber to PPS which is a material forming the insulating film, it is possible to improve the creep resistance of the insulating film. But it was impossible to sufficiently improve the creep resistance with the content of glass fiber sufficient to maintain injection moldabibity.
Also, since PPS is a resin having a glass transition point (Tg) at around 90xc2x0 C., at a higher temperature than Tg of PPS, the durability of the bearing comprising a PPS composition worsens.
An object of this invention is to provide an electrical pittingproof rolling bearing which has a good creep resistance of the insulating film formed on the bearing ring surface, in which even under high-temperature, high-load conditions, the interference of the bearing is stable with time, and which rotates smoothly.
Another object of this invention is to provide an electrical pittingproof rolling bearing in which even if it is used at a high temperature for a long time over the glass transition point of PPS, its insulating film is not creep-deformed under bearing load, and the interference of the bearing is stable with time, and which rotates smoothly.
It is also an object of this invention to provide an electrical pittingproof rolling bearing which can be used for a long time under large load and which can be used with a device for transmitting a rotating force from a motor of an electric railroad car.
In order to provide an electrical pittingproof rolling bearing in which the bearing interference is stable with time, according to this invention, there is provided an electrical pittingproof rolling bearing having bearing rings, the rolling bearing having an insulating film on the surface of the bearing ring, characterized in that the insulating film comprises a resin having a compressive permanent strain of 2% or less under heating/pressurizing conditions in which a pressure of 20 MPa is applied for 24 hours at 120xc2x0 C.
By forming the insulating film from a resin having a permanent compression strain of 2% or less under predetermined heating/pressurizing conditions, an electrical pittingproof rolling bearing is provided in which the interference of the bearing is sufficiently stable with time. Thus, even if it is repeatedly exposed to high temperatures, the size of the gap between the outer peripheral surface and the housing would not change, and tightening bolts for a ring for preventing the bearing from getting out would not loosen, so that the interference of the bearing is stable with time. A smoothly rotating electrical pittingproof bearing is thus provided.
Conversely, if the insulating film is formed from a resin having a permanent compression strain of over 2%, it may be necessary to rather thickly form the insulating film, or rattling may occur due to change in the interference of the bearing, so that wear tends to develop on the fitting surface.
Also, with an electrical pittingproof rolling bearing having an insulating film on a bearing ring surface, in order to solve the problem that the insulating film of a PPS resin deforms if it is used for a long time at a high temperature over the glass transition point of PPS resin, according to this invention, the insulating film is formed from a resin composition containing 30-80 vol % of polyphenylene sulfide resin (PPS) and 5-65 vol % of polyamide-imide resin.
A rolling bearing having an insulating film formed from a resin composition to which are added PPS and a polyamide-imide resin is superior in creep resistance particularly at high temperature to a conventional PPS-base insulating film. Thus an electrical pittingproof rolling bearing is provided in which the interference of the bearing is sufficiently stable with time.
With a rolling bearing having an insulating film formed from a resin composition to which is added a polyamide-imide resin by a predetermined amount, the resin composition has an injection-moldable fluidity in a molten state. Thus it is possible to mold an insulating film on a required portion of the bearing. Efficient manufacturing is thus possible.
According to the present invention, there is also provided an electrical pittingproof rolling bearing having bearing rings, the rolling bearing having an insulating film on the surface of the bearing ring, characterized in that the insulating film is formed from a resin composition containing 30-80 vol % of a polyphenylene sulfide resin, 5-65 vol % of a polyamide-imide resin, and 5-50 vol % of insulating inorganic materials. An electrical pittingproof rolling bearing having an insulating film comprising a PPS resin composition to which is added 5-50 vol % of an insulating inorganic material is particularly superior in creep resistance at high temperature. Also, the interference of the bearing is sufficiently stable with time.
With the electrical pittingproof rolling bearing according to this invention, the insulating film is molded to cover at least one of the outer peripheral surface of the outer ring and the inner peripheral surface of the inner ring, and the side faces of at least one of the inner and outer rings.
If such an electrical pittingproof rolling bearing is used in a motor of an electric railroad car or a mechanism for transmitting its rotating force or in both of them, even if a current flows through the rotary shaft of the motor, it will not leak to rails or metal parts forming another turning force transmission mechanism. Thus an electrical pittingproof rolling bearing is provided which has a reliable electrical pittingproofness.
The electrical pittingproof rolling bearing according to the present invention is used in a motor of an electric railroad car or in its rotating force transmitting mechanism.
Other features and objects of the present invention will become apparent from the following description made with reference to the accompanying drawings, in which: